In recent years, stretched films (i.e. heat-shrinkable films) composed of a polyvinyl chloride-based resin, a polystyrene-based resin or a polyester-based resin have come into wide use in packaging with a label serving to protect a glass bottle, a plastic bottle or the like and also display product information, cap sealing, assembly packaging, and so on. Among these heat-shrinkable films, polyvinyl chloride-based films have low heat resistance, and further, polyvinyl chloride-based films have problems that they generate a hydrogen chloride gas during incineration, cause generation of dioxin, and the like.
Polystyrene-based films have poor solvent resistance, so that an ink having a special composition must be used in printing, and further, polystyrene-based films have the problem that they are required to be incinerated at a high temperature, and thus a large amount of black smoke is generated along with an offensive smell during incineration. Thus, polyester-based heat-shrinkable films which have high heat resistance, are easily incinerated, and have excellent solvent resistance have come into wide use as shrinkable labels, and tended to be increasingly used with an increase in distribution amount of PET (polyethylene terephthalate) bottles etc.
However, conventional heat-shrinkable polyester-based films have been required to be further improved in their shrinkage property. Particularly, in comparison with a heat-shrinkable polystyrene-based film, a heat-shrinkable polyester-based film has the following problem: characters and pictures printed on the film may be distorted due to uneven shrinkage and generation of creases when a container such as a PET bottle, a polyethylene bottle or a glass bottle is covered with the film, and the film is shrunk. Users have desired that the distortion be reduced as much as possible.
In use of a heat-shrinkable film in covering processing of a container, pictures etc. are printed on the film as necessary, the film is then processed into the form of a label, a bag or the like, and attached to the container, and the label or the like is heat-shrunk, and attached to the container using a heating apparatus called a shrinkage tunnel. Such the shrinkage tunnel includes a steam tunnel which heat-shrink the film by blowing steam, and a hot air tunnel which heat-shrink the film by blowing hot air.
As compared to the hot air tunnel, the steam tunnel has higher heat transfer efficiency, and is capable of heat-shrinking the film more uniformly, so that a favorable finished external appearance can be obtained. However, polyester-based films have the problem that even when a steam tunnel is used, they are slightly inferior in finish property to polyvinyl chloride-based films and polystyrene-based films.
Further, when a polyester-based film is shrunk using a hot air tunnel in which temperature unevenness more easily occurs as compared to a steam tunnel, there is the problem that whitening in shrinkage, uneven shrinkage, generation of creases, distortion and so on easily occur, and thus the polyester-based film is inferior in finish property to a polyvinyl chloride-based film and a polystyrene-based film.
In view of the problems described above, for example, a method has been suggested in which a polyester-based elastomer is included in a polyester resin as a film raw material for improving the shrinkage finish property of a heat-shrinkable polyester-based film (Patent Document 1).
However, the heat-shrinkable polyester-based film described in Patent Document 1 has the problem that use of the film as a label for an easily thermally expandable bottle made of polyethylene or the like is not preferable from the viewpoint of performance and external appearance because even when the label is adhered to the bottle during heating and shrinkage, the bottle is cooled to approximately room temperature, so that the bottle expanded during heating returns to its usual size, and therefore the label is slacken. The heat-shrinkable polyester-based film in Patent Document 1 also has the problem that the film is hardly stretched in a longitudinal direction, a direction orthogonal to a traverse direction as a main shrinkage direction, and therefore has low mechanical strength in the longitudinal direction, and poor perforation opening property.
After application of Patent Document 1, the applicant of this application subsequently conducted studies on perforation opening property, and a heat-shrinkable polyester-based film having excellent perforation opening property as shown in Patent Document 2 was successfully provided.
However, there is still room for improvement in shrinkage finish property. In a hot air tunnel to be used in a preform for cap sealing, the size of equipment is often relatively small in accordance with its specifications, and there is the problem that finish property is deteriorated because a mold is not sufficiently cooled in a cooling step after a film passes through the tunnel. Thus, when the heat-shrinkable film in Patent Document 2 is used, the film is continuously shrunk by remaining heat in the mold (hereinafter, this phenomenon is referred to as secondary shrinkage), and therefore the preform loses its shape.